1. Field of the Invention
The embodiments of the invention generally relate to charge pumps and, more particularly, to an improved charge pump structure having high output resistance and a wide swing.
2. Description of the Related Art
The charge pump is an important circuit that is widely used in mixed signal systems. For example, in phase-locked loop circuits (PLL's), the charge pump is used to convert phase error detected by a phase/frequency detector (PFD) to analog voltage signals that are supplied to a voltage controlled oscillator. The charge pump consists of a charging current source and a discharging current sink. As a rule of thumb, it is important to ensure the current source and current sink have the same magnitude currents (e.g., in order to ensure optimal PLL performance). That is, large current mismatch between the components in the charge pump may degrade system performance, for instance, by introducing steady-state phase offset and increasing reference spurs in the PLL (See Y. S. Choi and D. H. Han, “Gain-Boosting Charge Pump for Current Matching in Phase-Locked Loop”, IEEE Trans. on Circuits and Systems-II Express Briefs, Vol. 53, No. 10, October 2006. pp. 1022-1025; J. S. Lee and M. S. Keel, S. Lim and S. Kim, “Charge Pump with Perfect Current Matching Characteristics in Phase-Locked Loop”, Electronics Letters, Vol. 36, No. 23, November 2000, pp. 1907-1908; B. Bahreyni and I. M. Filanovsky, “A 2.5-10-GHz Clock Multiplier Unit with 0.22 ps RMS Jitter in Standard 0.18 um CMOS”, IEEE J. Solid State Circuits, Vol. 39, No. 11, pp. 1862-1872, November 2004; and K. S. Ha and L. S. Kim, “Charge-Pump Reducing Current Mismatch in DLLs and PLLs”, Proc. IEEE International Symposium on Circuit and Systems (ISCAS), 2006). Moreover, such current magnitude variation may shift PLL loop bandwidth and also degrade PLL locking speed and jitter performance.